RT7285D
Feed-forward Capacitor (Cff)
Enable Operation (EN)
The RT7285D is optimized for ceramic output capacitors
and for low duty cycle applications. However for high-output
voltages, with high feedback attenuation, the circuit's
response becomes over-damped and transient response
can be slowed. In high-output voltage circuits (VOUT > 3.3V)
transient response is improved by adding a small “feed-
forward” capacitor (Cff) across the upper FB divider resistor
(Figure 1), to increase the circuit's Q and reduce damping
to speed up the transient response without affecting the
steady-state stability of the circuit. Choose a suitable
capacitor value that following below step.
For automatic start-up the EN pin can be connected to
VIN through a 100kΩ resistor. Its large hysteresis band
makes EN useful for simple delay and timing circuits. EN
can be externally pulled to VIN by adding a resistor-
capacitor delay (REN and CEN in Figure 2). Calculate the
delay time using EN's internal threshold where switching
operation begins (1.4V, typical).An external MOSFET can
be added to implement digital control of EN when no
system voltage above 2V is available (Figure 3). In this
case, a 100kΩ pull-up resistor, REN, is connected between
VINand the ENpin. MOSFET Q1 will be under logic control
to pull down the ENpin. To prevent enabling circuit when
VIN is smaller than the VOUT target value or some other
desired voltage level, a resistive voltage divider can be
placed between the input voltage and ground and
connected to ENto create an additional input under voltage
lockout threshold (Figure 4).
Get the BW the quickest method to do transient
response form no load to full load. Confirm the damping
frequency. The damping frequency is BW.
EN
R
EN
V
IN
EN
RT7285D
BW
C
EN
GND
V
OUT
Figure 2. External Timing Control
R
EN
100k
R1
C
ff
V
EN
RT7285D
GND
IN
FB
RT7285D
Q1
Enable
R2
GND
Figure 1. Cff Capacitor Setting
Figure 3. Digital Enable Control Circuit
Cff can be calculated base on below equation :
R
EN1
V
IN
EN
1
Cff =
2×3.1412×R1×BW ×0.8
R
EN2
RT7285D
GND
Internal Soft-Start (SS)
The RT7285D soft-start uses an internal soft-start time
Figure 4. ResistorDivider for Lockout Threshold Setting
800μs.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7285D-04 March 2018
www.richtek.com
11
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